This invention relates to the recovery of tungsten values from scrap cemented tungsten carbide.
Recovery of tungsten values from scrap tungsten carbide has been a problem in the carbide industry for a number of years. Cemented carbide tools are made by consolidating extremely hard and fine metal carbide particles together with a suitable binder or cement. Typically, such tools contain tungsten carbide cemented with cobalt although additional carbides such as the carbides of titanium, vanadium, chromium or molybdenum may also be present.
Cobalt is the most widely used cementing material although other cementing materials such as iron, nickel, chromium or molybdenum may be employed. Since all of the materials used in the cemented carbides are extremely valuable, it is desirable to reclaim the materials found therein.
Various processes have been used in the past with varying degrees of success. U.S. Pat. No. 3,953,194 to Hartline et al describes a process for reclaiming cemented metal carbide material by subjecting the metal carbide material to catastrophic oxidation to produce a mixture of metal oxide and the oxide of the cement. The resulting oxide is reduced and finally carbonized.
Another process described in U.S. Pat. No. 3,887,680 to MacInnis, Vanderpool and Boyer wherein tungsten carbide containing an iron group metal such as cobalt is oxidized from a friable oxidation product. The oxidation product is then ground and treated by digesting it in an aqueous solution of alkali metal hydroxide under controlled conditions to recover tungsten values.
Refractory carbides are a mixture of from 4-20% Co, which acts as a binder, with 96-80% carbide. Additives, such as MoC, Vc, TiC, TaC, and NbC may be added to impart certain advantages to the carbide. To recover the valued metals from these carbides, they are oxidized in air or oxygen. The oxidation removes the carbon leaving the W, Ti, Mo, etc. as oxides; which can be further processed to recover the valued metals.
These regular carbides, however, sometimes fail during use because of poor oxidation resistance. The cutting edges become hot and oxidize and therefore become dull. To overcome this, the carbide industry has coated the base carbides with coatings which impart more oxidation resistance to these pieces. These coatings are Al.sub.2 O.sub.3, TiC and TiN or mixtures of these. They are also putting on multiple coatings such as an inner layer of TiC, followed by a second layer of Al.sub.2 O.sub.3, then a final outside layer of TiN. When these carbides are fed through the regular air oxidation recovery processes, the TiN and TiC will oxidize if they are not covered with the Al.sub.2 O.sub.3 type coating. However, the Al.sub.2 O.sub.3 type coating is virtually unaffected.